One document matched: draft-ietf-aaa-transport-01.txt
Differences from draft-ietf-aaa-transport-00.txt
AAA Working Group Bernard Aboba
INTERNET-DRAFT Microsoft
Category: Standards Track
<draft-ietf-aaa-transport-01.txt>
21 February 2001
AAA Transport Profile
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Task
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or to cite them other than as "work in
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
1. Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
2. Abstract
This document discusses transport issues that arise with protocols for
Authentication, Authorization and Accounting. It also provides
recommendations on the use of transport by AAA protocols. This includes
usage of standards-track RFCs as well as experimental proposals.
3. Introduction
This document discusses transport issues that arise with protocols for
Authentication, Authorization and Accounting. It also provides
recommendations on the use of transport by AAA protocols. This includes
usage of standards-track RFCs as well as experimental proposals.
3.1. Requirements language
In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
"recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
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described in [1].
3.2. Terminology
Accounting
The act of collecting information on resource usage for the
purpose of trend analysis, auditing, billing, or cost
allocation.
Administrative Domain
An internet, or a collection of networks, computers, and
databases under a common administration. Computer entities
operating in a common administration may be assumed to share
administratively created security associations.
Attendant A node designed to provide the service interface between a
client and the local domain.
Authentication
The act of verifying a claimed identity, in the form of a pre-
existing label from a mutually known name space, as the
originator of a message (message authentication) or as the
end-point of a channel (entity authentication).
Authorization
The act of determining if a particular right, such as access
to some resource, can be granted to the presenter of a
particular credential.
Billing The act of preparing an invoice.
Broker A Broker is an entity that is in a different administrative
domain from both the home AAA server and the local ISP, and
which provides services, such as facilitating payments between
the local ISP and home administrative entities. There are two
different types of brokers; proxy and routing.
Client A node wishing to obtain service from an attendant within an
administrative domain.
End-to-End
End-to-End is the security model that requires that security
information be able to traverse, and be validated even when an
AAA message is processed by intermediate nodes such as
proxies, brokers, etc.
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Foreign Domain
An administrative domain, visited by a Mobile IP client, and
containing the AAA infrastructure needed to carry out the
necessary operations enabling Mobile IP registrations. From
the point of view of the foreign agent, the foreign domain is
the local domain.
Home Domain
An administrative domain, containing the network whose prefix
matches that of a mobile node's home address, and containing
the AAA infrastructure needed to carry out the necessary
operations enabling Mobile IP registrations. From the point
of view of the home agent, the home domain is the local
domain.
Hop-by-hop
Hop-by-hop is the security model that requires that each
direct set of peers in a proxy network share a security
association, and the security information does not traverse a
AAA entity.
Inter-domain Accounting
Inter-domain accounting is the collection of information on
resource usage of an entity within an administrative domain,
for use within another administrative domain. In inter-domain
accounting, accounting packets and session records will
typically cross administrative boundaries.
Intra-domain Accounting
Intra-domain accounting is the collection of information on
resource within an administrative domain, for use within that
domain. In intra-domain accounting, accounting packets and
session records typically do not cross administrative
boundaries.
Local Domain
An administrative domain containing the AAA infrastructure of
immediate interest to a Mobile IP client when it is away from
home.
Proxy A AAA proxy is an entity that acts as both a client and a
server. When a request is received from a client, the proxy
acts as a AAA server. When the same request needs to be
forwarded to another AAA entity, the proxy acts as a AAA
client.
Local Proxy
A Local Proxy is a AAA server that satisfies the definition of
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a Proxy, and exists within the same administrative domain as
the network device (e.g. NAS) that issued the AAA request.
Typically, a local proxy will enforce local policies prior to
forwarding responses to the network devices, and are generally
used to multiplex AAA messages from a large number of network
devices.
Network Access Identifier
The Network Access Identifier (NAI) is the userID submitted by
the client during network access authentication. In roaming,
the purpose of the NAI is to identify the user as well as to
assist in the routing of the authentication request. The NAI
may not necessarily be the same as the user's e-mail address
or the user-ID submitted in an application layer
authentication.
Routing Broker
A Routing Broker is a AAA entity that satisfies the definition
of a Broker, but is NOT in the transmission path of AAA
messages between the local ISP and the home domain's AAA
servers. When a request is received by a Routing Broker,
information is returned to the AAA requester that includes the
information necessary for it to be able to contact the Home
AAA server directly. Certain organizations providing Routing
Broker services MAY also act as a Certificate Authority,
allowing the Routing Broker to return the certificates
necessary for the local ISP and the home AAA servers to
communicate securely.
Non-Proxy Broker
A Routing Broker is occasionally referred to as a Non-Proxy
Broker.
Proxy Broker
A Proxy Broker is a AAA entity that satisfies the definition
of a Broker, and acts as a Transparent Proxy by acting as the
forwarding agent for all AAA messages between the local ISP
and the home domain's AAA servers.
Real-time Accounting
Real-time accounting involves the processing of information on
resource usage within a defined time window. Time constraints
are typically imposed in order to limit financial risk.
Roaming Capability
Roaming capability can be loosely defined as the ability to
use any one of multiple Internet service providers (ISPs),
while maintaining a formal, customer-vendor relationship with
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only one. Examples of cases where roaming capability might be
required include ISP "confederations" and ISP- provided
corporate network access support.
Transparent Proxy
A Transparent Proxy is a AAA server that satisfies the
definition of a Proxy, but does not enforce any local policies
(meaning that it does not add, delete or modify attributes or
modify information within messages it forwards).
4. Issues in AAA transport usage
Issues that arise in AAA transport usage include:
Application-driven versus network-driven behavior
Slow failover
Use of Nagle Algorithm
Multiple connections
Connection load balancing
Duplicate detection
Invalidation of transport parameter estimates
Inability to use fast re-transmit
Head of line blocking
Congestion avoidance
Delayed acknowledgments
Premature Failover
We discuss each of these issues in turn.
4.1. Application-driven versus network-driven behavior
Steady state AAA transport behavior is typically application rather than
network driven. For example, a 48-port NAS with an average session time
of 20 minutes will on average send only 144 authentication/authorization
requests/hour, and an equivalent number of accounting requests. This
translates to an average inter-packet spacing of 25 seconds.
Even on much larger NAS devices, the inter-packet spacing is often
larger than the Round Trip Time (RTT). For example, a 2048-port NAS with
an average session time of 10 minutes will on average send 3.4
authentication/authorization requests/second, and an equivalent number
of accounting requests. This translates to an average inter-packet
spacing of 293 ms.
Note that transient behavior can result in much lower inter-packet
spacing. For example, after a NAS reboot previously stored accounting
records may be sent to the accounting server in rapid succession.
Similarly, after recovery from a power failure, users may respond with a
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large number of simultaneous logins. Thus while application-driven AAA
transport behavior is the norm, there are situations in which behavior
may be network driven.
Note that even with high inter-packet spacings as seen by the NAS, it is
possible for AAA clients and servers to experience congestion, even in
the absence of any other traffic. For example, while a given AAA client
may not send substantial traffic, many AAA clients may interact with a
given AAA proxy or server. Thus routers close to a heavily loaded proxy
or server may experience congestion, even though traffic close to the
client is very light. For example, if 10,000 48-ports NASes were to use
the same AAA proxy or server, that proxy or server would receive 400
authentication/authorization requests/second and an equivalent number of
accounting requests. For 1000 octet requests, this could generate as
much as 6.4 Mbps of incoming traffic at the AAA proxy or server.
While such a transaction rate is within the capabilities of the fastest
AAA servers and proxies, implementations exist that cannot handle such a
high load, and thus high queuing delays and/or dropped packets may be
experienced at the server, even if the routers on the path are not
congested. Thus, a well designed AAA protocol needs to be able to handle
congestion occurring at the AAA server, as well as congestion
experienced within the network.
4.2. Slow failover
Where TCP [5] is used as the transport, aAAA implementations waiting
until a TCP connection times out before failing over will experience
very slow failover times. This is not an issue for SCTP [6], which
enables adjustment of the failover timer at the transport layer.
While AAA implementations can use application failover timers, without
implementation of Congestion Manager [19] or application-layer
heartbeats, there is no way to adapt these timers to the transport
parmeters in use in a particular situation.
4.3. Use of Nagle Algorithm
AAA protocol messages are often smaller than the maximum segment size
(MSS). While exceptions occur when certificate-based authentication
issued or where a low path MTU is found, typically AAA protocol messages
are less than 1000 octets. Therefore, the total packet count, and
associated network overhead can be reduced by combining multiple AAA
messages within a single packet. While this does not reduce the work
required by the application in parsing packets and responding to the
messages, it does reduce the number of packets processed by routers
along the path.
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However, within the application-driven regime, the NAS will typically
receive a reply from the home server prior to having another request to
send. This implies, for example, that accounting requests will typically
be sent individually rather than being batched by the transport layer.
As a result, within the application-driven regime, the Nagle algorithm
[12] is ineffective.
4.4. Multiple connections
Since the RADIUS [2] Identifier field is a single octet, a maximum of
256 requests can be in progress between two endpoints described by a
5-tuple: (NAS IP address, NAS port, UDP, RADIUS server IP address,
RADIUS server port). In order to get around this limitation, NAS
implementations of RADIUS have utilized more than one sending port. In
fact, some implementations have used a different sending port for each
NAS port.
Were this behavior to be extended to AAA protocols operating over
reliable transport, the result would be multiplication of the effective
slow-start rampup by the number of connections. For example, if a NAS
had ten connections open to a AAA proxy, and used a per-connection
initial window [20] of 2, then the effective initial window would be 20.
This is inappropriate, since it would permit the NAS to send a large
burst of packets into the network.
4.5. Connection load balancing
RADIUS [2] supported the concept of primary and secondary servers, in
order to support failover. Since all traffic will typically flow go to
the primary server unless it is unavailable, the secondary server
typically remains unused.
In order to lessen queuing delays and ameliorate the head of line
blocking problem, it is desirable for a AAA protocol to be able to load
balance between multiple connections. While sophisticated load balancing
techniques are possible, substantial benefits can be achieved by use of
static load balancing. In static load balancing, traffic is distributed
between servers based on static "weights" corresponding to server
capacity.
4.6. Duplicate detection
In order to avoid spurious re-transmits, it is necessary for TCP [24]
and SCTP [6] to include logic for estimating the re-transmission timer.
However, even with a good RTO estimator, RTT distributions are typically
heavy-tailed so that there will be some number of false re-transmits. As
a result, AAA servers must be prepared to receive duplicate requests,
and it is typical for server implementations to cache responses so as to
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make it possible respond to such duplicate requests more efficiently.
4.7. Invalidation of transport parameter estimates
Congestion control principles [9],[16] require the ability of a
transport protocol to respond effectively to congestion, as sensed via
increasing delays, packet loss, or explicit congestion notification.
With network-driven applications, it is possible to respond to
congestion on a timescale comparable to the round-trip time (RTT).
However, with application-driven AAA protocols, the time between sends
may be considerably larger than the RTT, so that the network conditions
can not be assumed to persist between sends. For example, the congestion
window may grow during a period in which congestion is being
experienced, because few packets are sent, limiting the opportunity for
feedback. Similarly, after congestion is detected, the congestion window
may remain small, even though the network conditions that existed at the
time of congestion no longer apply by the time when the next packets are
sent. In addition, due to the low sampling interval, estimates of RTT
and RTO may become invalid.
4.8. Inability to use fast re-transmit
When congestion window validation [13] is implemented, the result is
that AAA protocols operate much of the time in slow-start with an
initial congestion window set to 1 or 2, depending on the implementation
[20]. This implies that AAA protocols gain little benefit from the
windowing features of reliable transport.
Since the congestion window is so small, it is generally not possible to
receive enough duplicate ACKs (3) to trigger fast re-transmit. As a
result, dropped packets will require a retransmission timeout (RTO).
4.9. Head of line blocking
Head of line blocking occurs during periods of packet loss where the
time between sends is shorter than the Re-transmission timeout value
(RTO). In such situations, packets back up in the send queue until the
lost packet can be successfully re-transmitted.
Head of line blocking is typically only an issue on larger NASes. For
example, a 48-port NAS with an average inter-packet spacing of 25
seconds is unlikely to have an RTO greater than this unless severe
packet loss is experienced. However, a 2048-port NAS with an average
inter-packet spacing of 293 ms may experience head-of-line blocking
since the inter-packet spacing is less than the minimum RTO value of 1
second.
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4.10. Congestion avoidance
The law of conservation of packets [9] suggests that a client should not
send another packet into the network until it can be reasonably sure
that a packet has exited the network on the same path. In the case of a
AAA client, the law suggests that it should not retransmit to the same
server or choose another server until it can be reasonably sure that a
packet has exited the network on the same path. If the client advances
the window as responses arrive, then the client will "self clock",
adjusting its transmission rate to the available bandwidth.
While a AAA client using a reliable transport such as TCP [5] or SCTP
[6] will self-clock when communicating directly with a AAA-server, end-
to-end self-clocking is not assured when a AAA proxy is present.
For example, let us consider the situation where a bottleneck exists
between the AAA proxy and a AAA server. In this situation, self-clocking
will exist between the AAA client and AAA proxy. However, since no
direct communication occurs between the AAA client and AAA server, there
is no way for the AAA client to estimate the end-to-end transport
parameters, or to adjust its sending rate to the bottleneck bandwidth
between the AAA proxy and AAA server.
Moreover, in this situation the end-to-end performance will be
determined by aspects of the proxy implementation. As described in the
Appendix, AAA proxies may be classified as Re-directs, Store and Forward
Proxies, Application layer Proxies, and Transport proxies. Of these
proxies, only the Transport and Re-direct proxy types result in direct
communication between the AAA client and AAA server. Thus for these
proxy types, end-to-end self-clocking will occur.
However when store and forward or application layer proxies are used,
the result is the presence of two separate and de-coupled transport
connections, one between the AAA client and proxy, and other between the
AAA proxy and server. Since the two transport connections are de-
coupled, transport layer ACKs do not flow end-to-end, and self-clocking
does not occur. The result is that end-to-end transport performance in
the presence of proxies will always be worse in terms of delay and
packet loss than if the AAA client and server were communicating
directly.
Methods to induce coupling between the two transport connections are
difficult to implement. For example, if the proxy operates with a large
receive buffer, it is possible that a large queue will develop on the
receiving side, since the AAA client is able to send packets to the AAA
proxy more rapidly than the proxy can send them to the AAA server.
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One possible solution is for the AAA proxy to operate with a receive
buffer that is no larger than its send buffer. If this is done, "back
pressure" (closing of the receive windows) will be exerted by the proxy
on the AAA client at times when the proxy send buffer fills. However,
unless multiple connections exist between the AAA client and AAA proxy,
the use of "back pressure" will be somewhat indiscriminate. This is
because presence of a bottleneck in sending to a particular AAA server
may result in closing of the receive window to all traffic sent by the
AAA client, even traffic destined to AAA servers where no bottleneck
exists.
Since multiple connections between a AAA client and proxy result in
multiplication of the effective slow-start ramp rate, this is not
recommended. As a result, use of "back pressure" appears impractical for
AAA.
4.11. Delayed Acknowledgments
As described in Appendix A, ACKs may comprise as much as half of the
traffic generated in a AAA exchange. This occurs because AAA
conversations are typically application-driven, and therefore there is
frequently not enough traffic to enable ACK piggybacking. ACK
parameters (such as the value of the delayed ACK timer) are typically
fixed by the TCP implementation and are not tunable by the application.
Thus, the use of TCP or SCTP transport by AAA protocols may result in
doubling of traffic over what would be experienced with UDP transport.
4.12. Premature failover
RADIUS [2] supported the concept of primary and secondary servers, in
order to support failover. However, the failover algorithm was never
specified. As a result, RADIUS failover implementations vary in quality,
with some failing over prematurely, violating the law of "conservation
of packets".
In the presence of application layer proxies, a NAS or proxy has no
direct connection a AAA server, and is unable to estimate the end-to-end
transport parameters. As a result, a NAS r proxy awaiting an
application-layer response from the server has no transport-based
mechanism for determining an appropriate failover timer.
For example, if the path between the AAA proxy and server includes a
high delay link, it is possible that the NAS will failover to another
proxy while packets are still in flight. This violates the principle of
"conservation of packets" since the NAS will inject additional packets
into the network before having evidence that a previously sent packet
has left the network. Such behavior can result in worsening the
situation on an already congested link, resulting in congestive collapse
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[9].
5. AAA transport profile
In order to address the issues described previously, it is recommended
that AAA protocols make use of standards track as well as experimental
techniques. Recommendations on AAA transport usage are described below.
5.1. Transport mappings
AAA Servers MUST support TCP & SCTP. NASes MUST support TCP, and MAY
support SCTP. As support for SCTP improves, it is possible that SCTP
support will be required on NASes at some point in the future. TCP is
required on NASes because not all NASes have SCTP in their protocol
stacks, and because existing firewalls may not support SCTP.
5.2. Slow failover
In order to enable AAA implementations to more quickly detect transport
and application-layer failures, AAA protocols MUST support an
application layer heartbeat. The heartbeat interval MUST NOT be less
than MINIMUM_HEARTBEAT_INTERVAL (3 seconds).
Where Congestion Manager [19] is available, AAA implementations SHOULD
adapt the application-layer heartbeat parameters using the algorithm
described in [6].
5.3. Use of Nagle Algorithm
While AAA protocols typically operate in the application-driven regime,
there are circumstances in which they are network driven. For example,
where a NAS reboots, or where connectivity is restored between a NAS and
a AAA proxy, it is possible that multiple packets will be available for
sending.
As a result, there are circumstances where the transport-layer batching
provided by the Nagle Algorithm (12) is useful, and as a result, AAA
implementations MUST enable the Nagle algorithm, RFC 896 [12].
5.4. Multiple connections
5.5. AAA protocols SHOULD use only a single persistent connection
between a AAA client and a AAA proxy or server, and SHOULD provide for
pipelining of requests, so that more than one request can be in progress
at a time. In order to minimize use of inactive connections in roaming
situations, a AAA proxy MAY bring down a connection to a AAA server if
the connection has been un-utilized (discounting the heartbeat) for a
certain period of time, which MUST NOT be less than BRINGDOWN_INTERVAL
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(5 minutes).
In the event that a connection goes down to a given AAA proxy or server,
the AAA client MAY attempt to bring it back up periodically. However,
these attempts to revive the connection MUST NOT be more aggressive than
the HEARTBEAT_MINIMUM (3 seconds).
5.6. Connection load balancing
In order to support failover and failback, a AAA implementation MUST
support connection failure detection, and MUST NOT send packets on a
socket that it knows to be inoperative. This implies that the "weight"
on a non-operable connection MUST be reduced to zero.
In order to provide additional resilience and address head of line
blocking issues, a AAA client MAY maintain connections between multiple
AAA proxies, and a AAA proxy MAY maintain connections between multiple
AAA servers. A AAA client/proxy connected to multiple proxies/servers
can treat them as primary/secondary or balance load between them. It is
recommended that static load balancing SHOULD be supported using
Pearson's hash [29] applied to the NAI [28]. Hashing on the NAI ensures
that traffic for a given destination will be sent to the same proxy,
maximizing use of the routing cache. More sophisticated load balancing
techniques, such as dynamic load balancing, MAY also be supported by AAA
clients and proxies.
5.7. Duplicate detection
AAA protocols MUST support an end-to-end message identifier, to enable
the home server to detect duplicates. Hop-by-hop identifiers whose value
may change at each hop are not sufficient, since a AAA server may
receive the same message from multiple proxies. For example, a AAA
client can send a request to Proxy1, then failover and resend the
request to Proxy2; both proxies forward the request to the home AAA
server, with different hop-by-hop identifiers. A Session-ID is
insufficient as it does not distinguish different messages for the the
same session.
5.8. Invalidation of transport parameter estimates
In order to address invalidation of transport parameter estimates, AAA
protocol implementations MAY utilize Congestion Window Validation (RFC
2861) [13] and RTO Validation [30].
RFC 2581 [14] recommends that a connection go into slow-start after a
period where no traffic has been sent within the RTO interval. RFC 2861
[13] recomends only increasing the congestion window if it was full when
the ACK arrived. The congestion window is reduced by half once every RTO
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interval if no traffic is received.
When Congestion Window Validation is used, the congestion window will
not build during application-driven periods, and instead will be
decayed. As a result, AAA applications operating within the application-
driven regime will typically run with a congestion window equal to the
initial window [21] much of the time. This implies that AAA protocols
will typically operate in "perpetual slowstart".
During periods in which AAA behavior is application-driven this will
have no effect, since the time between packets will be larger than RTT,
and thus AAA will operate with an effective congestion window of 1.
However, during network-driven periods, the effect will be to space out
sending of AAA packets. Thus instead of being able to send a large burst
of packets into the network, a NAS will need to wait several RTTs as the
congestion window builds during slow-start.
For example, a NAS operating with an initial window of 2, with 35 AAA
requests to send would take approximately 6 RTTs to send them, as the
congestion window builds during slow start: 2, 3, 3, 6, 9, 12. After the
backlog is cleared, the implementation will once again be application-
driven and the congestion window size will decay.
Note that RFC 2861 [13] does not address the issue of RTO validation.
This is also a problem, particularly when the Congestion Manager [19] is
implemented. During periods of high packet loss, the RTO may be
repeatedly increased via exponential backoff, and may attain a high
value. Due to lack of timely feedback on RTT and RTO during application-
driven periods, the high RTO estimate may persist long after the
conditions that generated it have dissipated.
In order to address this issue, an RTO validation procedure is required.
The following procedure [30] is recommended, and will be documented in
the form of an Internet-Draft at some point in the future:
After the congestion window is decayed according to [13], reset the
estimated RTO to 3 seconds. After the next packet comes in, re-calculate
RTTavg, RTTdev, and RTO according to the method described in [14].
5.9. Inability to use fast re-transmit
When Congestion Window Validation (RFC 2861) [13] is used, AAA
implementations will operate with a congestion window equal to the
initial window much of the time. As a result, the window size will often
not be large enough to enable use of fast re-transmit.
To address this issue, AAA implementations SHOULD implement Limited
Transmit, as described in RFC 3042 [21]. Rather than reducing the number
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of duplicate ACKs required for triggering fast recovery, which would
increase the number of inappropriate re-transmissions, Limited Transmit
enables the window size be increased, thus enabling the sending of
additional packets which in turn may trigger fast re-transmit without a
change to the algorithm.
However, if congestion window validation [13] is implemented, this
proposal will only have an effect in situations where the time between
packets is less than the estimated retransmission timeout (RTO). If the
time between packets is greater than RTO, additional packets will
typically not be available for sending so as to take advantage of the
increased window size. As a result, AAA protocols will typically operate
with the lowest possible congestion window size, resulting in a re-
transmission timeout for every lost packet.
5.10. Head of line blocking
The head-of-line blocking problem can be addressed by a combination of
Limited Transmit [21], and connection load balancing.
5.11. Congestion avoidance
In order to improve upon default timer estimates, AAA implementations
MAY implement the Congestion Manager (CM) [19]. CM is an end-system
module that:
(i) Enables an ensemble of multiple concurrent streams from a sender
destined to the same receiver and sharing the same congestion
properties to perform proper congestion avoidance and control, and
(ii) Allows applications to easily adapt to network congestion.
The CM helps integrate congestion management across all applications and
transport protocols. The CM maintains congestion parameters (available
aggregate and per-stream bandwidth, per-receiver round-trip times, etc.)
and exports an API that enables applications to learn about network
characteristics, pass information to the CM, share congestion
information with each other, and schedule data transmissions.
The CM enables the AAA application to access transport parameters
(RTTavg, RTTdev) via callbacks. RTO estimates are currently not
available via the callback interface, though they probably should be.
Where available, transport parameters SHOULD be used to improve upon
default timer values.
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5.12. Premature Failover
To prevent premature failover, all AAA messages sent by a AAA client or
proxy (including accounting) MUST include an a maximum wait time. If the
next hop server cannot return the reply within that time period, it MUST
send an error message with an appropriate reason code. The maximum wait
time MUST NOT be shorter than MINIMUM_WAIT_INTERVAL (15 seconds).
Application Layer error messages are needed, so that NAS can do
appropriate failover. Failures can occur at both the transport and
application layers; for example, the NAS-proxy or Proxy-AAA server
transport connections can fail or a proxy/AAA server can be congested or
busy. At the application layer, the AAA application can fail. In order
to enable proper failover behavior, the NAS or proxy must be able to
distinguish between these conditions.
The following Application Layer Status Messages are recommended:
"Busy": Proxy/Server too busy to handle additional requests, NAS
should failover all requests to another proxy/server.
"Forwarding": Proxy has located AAA server, but timely response is
not forthcoming; NAS should reset application layer timers, wait for final
response.
"Can't Locate": Proxy can't locate the AAA server for the indicated
realm; NAS should failover that request to another proxy.
"Failover": Proxy has tried primary server, is failing over to
secondary server; NAS should reset application layer timers, wait for final
response.
"Can't Forward": Proxy has tried both primary and secondary AAA
servers with no response; NAS should failover to another proxy.
"Processing": Server cannot provide an immediate response to this
request; NAS should failover this request to another server, but not
all requests.
These messages differ in that some tell the NAS that the proxy/server is
too busy for any request and therefore that the connection should come
down for a while; some say that the proxy/AAA server can't handle a
particular request, implying failover for that request alone; some
indicate that the ultimate destination cannot be reached or isn't
responding, implying per-request failover. Note that these messages are
all hop-by-hop.
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6. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Rigney, C., Willens, S., Rubens, A., Simpson, W., "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[3] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[4] Calhoun, P., Rubens, A., Akhtar, H., Guttman, E., "DIAMETER Base
Protocol", Internet draft (work in progress), draft-ietf-aaa-
diameter-00.txt, February 2001.
[5] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[6] R. Stewart et al., "Stream Control Transmission Protocol", RFC
2960, October 2000.
[7] Aboba, B., Vollbrecht, J., "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[8] Aboba, B., Arkko, J., "Introduction to Accounting Management", RFC
2985, June 2000.
[9] Jacobson, V., "Congestion Avoidance and Control", Computer
Communications Review, ACM SIGCOMM,
[10] Blunk, L. and J. Vollbrecht, "PPP Extensible Authentication
Protocol (EAP)", RFC 2284, March 1998.
[11] Rigney, C., Willats, W., Calhoun, P., "RADIUS Extensions", RFC
2869, June 2000.
[12] Nagle, J., "Congestion Control in IP/TCP", RFC 896, January 1984.
[13] Handley, M., Padhye, J., Floyd, S., "TCP Congestion Window
Validation", RFC 2861, June 2000.
[14] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion Control",
RFC 2581, April 1999.
[15] Paxson, V., Allman, M., Dawson, S., Fenner, W., Griner, J.,
Heavens, I., Lahey, K., Semke, J. and B. Volz, "Known TCP
Implementation Problems", RFC 2525, March 1999.
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[16] Floyd, S., "Congestion Control Principles", RFC 2914, September
2000.
[17] Dawkins, S., Montenegro, G., Kojo, M. and V. Magret, "End-to-end
Performance Implications of Slow Links", Internet draft (work in
progress), draft-ietf-pilc-slow-04.txt, July 2000.
[18] Jacobson, V., Braden, R. and D. Borman, "TCP Extensions for High
Performance", RFC 1323, May 1992.
[19] Balakrishnan, H., Seshan, S., "The Congestion Manager", Internet
draft (work in progress), draft-ietf-ecm-cm-03.txt, November 2000.
[20] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's Initial
Window", RFC 2414, September 1998.
[21] Allman, M., Balakrishnan H., Floyd, S., "Enhancing TCP's Loss
Recovery Using Limited Transmit", RFC 3042, January 2001.
[22] Matt Mathis, Jamshid Mahdavi, Sally Floyd, Allyn Romanow. "TCP
Selective Acknowledgment Options", RFC 2018, October 1996.
[23] Floyd, S., Henderson, T., "The NewReno Modification to TCP's Fast
Recovery Algorithm", RFC 2582, April 1999.
[24] Paxson, V., Allman, M., "Computing TCP's Retransmission Timer", RFC
2988, November 2000.
[25] Floyd, S., Mahdavi, J., Mathis, M., Podolsky, M., Romanow, A., "An
Extension to the Selective Acknowledgment (SACK) Option for TCP",
RFC 2883, July 2000.
[26] Montenegro, G., Dawkins, S., Kojo, M., Magret, V., Vaidya, N.,
"Long Thin Networks", RFC 2757, January 2000.
[27] Touch, J., "TCP Control Block Interdependence", RFC 2140, April
1997.
[28] Aboba, B. and M. Beadles, "The Network Access Identifier", RFC
2486, January 1999.
[29] Volz, B., Gonczi, S., Lemon, T., Stevens, R., "DHC Load Balancing
Algorithm", Internet-draft (work in progress), draft-ietf-dhc-
loadb-03.txt, September 2000.
[30] Allison Mankin, personal communication.
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7. Appendix A - AAA proxy bestiary
As described in [2],[7] proxies have become a common feature of the AAA
landscape in order to support services such as roaming and shared use
networks. Such proxies are used both for authentication/authorization,
as well as accounting [8].
AAA proxies come in several varieties, including:
Application-layer proxies
Re-directs
Store and Forward proxies
Transport layer proxies
The transport layer behavior of each of these proxies is described in
turn.
7.1. Application-layer proxies
A conventional application-layer AAA proxy does not respond to a NAS
request until it receives a response from the AAA server. Since the
Nagle algorithm is typically not triggered in AAA exchanges, the typical
behavior of an application-layer AAA proxy operating over reliable
transport within the application-driven regime is show below.
Time NAS Proxy Home Server
------ --- ----- -----------
0 Request
------->
OTTnp + Tpr Request
------->
OTTnp + TdA Delayed ACK
<-------
OTTnp + OTTph + Reply/ACK
Tpr + Tsr <-------
OTTnp + OTTph +
Tpr + Tsr + Reply
OTThp + TpR <-------
OTTnp + OTTph +
Tpr + Tsr + Delayed ACK
OTThp + TdA ------->
OTTnp + OTTph +
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OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TdA ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Proxy)
OTTpn = One-way trip time (Proxy to NAS)
OTTph = One-way trip time (Proxy to Home server)
OTThp = One-way trip time (Home Server to Proxy)
TdA = Delayed ACK timer
Tpr = Proxy request processing time
TpR = Proxy reply processing time
Tsr = Server request processing time
With application-layer proxies two connections are established, one from
the NAS to the AAA proxy, and another from the AAA proxy to the AAA
server. Since these connections are de-coupled, the end-to-end
conversation between the NAS and AAA server will not self clock.
Another thing to notice about this situation is that ACKs may comprise
as much as half of the traffic. This occurs because ACK parameters
(such as the value of the delayed ACK timer) are typically fixed by the
TCP implementation and are not tunable by the application. Since AAA
traffic is application-driven, there is frequently not enough traffic to
enable ACK piggybacking. Thus, the use of reliable transport by AAA
protocols may result in as much as a doubling of traffic over what would
be experienced with UDP transport.
A detailed examination of the trace reveals the conditions under which
this may occur. At time 0, the NAS sends a request to the proxy.
Ignoring the serialization time, the request arrives at the proxy at
time OTTnp, and the proxy takes an additional Tpr in order to forward
the request toward the home server. At time TdA after receiving the
request, the proxy sends a delayed ACK. The delayed ACK is sent, rather
than being piggybacked on the reply, as long as TdA < OTTph + OTThp +
Tpr + Tsr + TpR.
Typically Tpr < TdA, so that the delayed ACK is sent after the proxy
forwards the request toward the home server, but before the proxy
receives the reply from the server. However, depending on the TCP
implementation on the proxy and when the request is received, it is also
possible for the delayed ACK to be sent prior to forwarding the
request.
At time OTTnp + OTTph + Tpr, the server receives the request, and Tsr
later it generates the reply. Where Tsr < TdA, the reply will contain a
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piggybacked ACK. However, depending on the responsiveness of the AAA
server and the server's TCP implementation, it is conceivable that the
ACK and reply will be sent separately. This may be the case, for
example, where a slow database or filestore must be consulted by the
server prior to sending the reply.
At time OTTnp + OTTph + OTThp + Tpr + Tsr the reply/ACK reaches the
proxy, which then takes TpR additional time to forward the reply to the
NAS. At TdA after receiving the reply, the proxy generates a delayed
ACK. Typically TpR < TdA so that the delayed ACK is sent to the server
after the proxy forwards the reply to the NAS. However, depending on the
circumstances and the proxy TCP implementation, the delayed ACK may be
sent first. As in the case of the delayed ACK sent in response to a
request, which may be piggybacked if the reply can be received quickly
enough, piggybacking of the ACK sent in response to a reply from the
server is only possible if additional request traffic is available to
piggyback on. However, due to the high inter-packet spacings in typical
AAA scenarios, this is unlikely unless the AAA protocol supports a reply
ACK.
At time OTTnp + OTTph + OTThp + OTTpn + Tpr + Tsr + TpR the NAS receives
the reply. TdA later, a delayed ACK is generated.
7.2. Re-directs
Re-directs operate by referring a NAS to the AAA server, enabling the
NAS to talk to the AAA server directly. The sequence of events is as
follows:
Time NAS Re-direct Home Server
------ --- --------- -----------
0 Request
------->
OTTnp + Tpr Redirect/ACK
<-------
OTTnp + Tpr + Request
OTTpn + Tnr ------->
OTTnp + OTTpn +
Tpr + Tsr + Reply/ACK
OTTns <-------
OTTnp + OTTpn +
OTTns + OTTsn +
Tpr + Tsr + Delayed ACK
TdA ------->
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Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Re-direct)
OTTpn = One-way trip time (Re-direct to NAS)
OTTns = One-way trip time (NAS to Server)
OTTsn = One-way trip time (Server to NAS)
TdA = Delayed ACK timer
Tpr = Re-direct processing time
Tnr = NAS re-direct processing time
Tsr = Server request processing time
Since with re-directs a direct transport connection is established
between the NAS and the AAA server, the end-to-end connection will self-
clock.
Delayed ACKs are also reduced as compared with the application-layer
proxy case since the Re-direct and Home Server will typically be able to
piggyback replies with the ACKs.
7.3. Store and Forward proxies
With a store and forward proxy, the proxy may send a reply to the NAS
prior to forwarding the request to the server. While store and forward
proxies are most frequently deployed for accounting [8], they also can
be used to implement authentication/authorization policy, as described
in [7]. With a store and forward proxy, the sequence of events is as
follows:
Time NAS Proxy Home Server
------ --- ----- -----------
0 Request
------->
OTTnp + TpR Reply/ACK
<-------
OTTnp + Tpr Request
------->
OTTnp + OTTph + Reply/ACK
Tpr + Tsr <-------
OTTnp + OTTph +
Tpr + Tsr + Reply
OTThp + TpR <-------
OTTnp + OTTph +
Tpr + Tsr + Delayed ACK
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OTThp + TdA ------->
OTTnp + OTTph +
OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TdA ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Proxy)
OTTpn = One-way trip time (Proxy to NAS)
OTTph = One-way trip time (Proxy to Home server)
OTThp = One-way trip time (Home Server to Proxy)
TdA = Delayed ACK timer
Tpr = Proxy request processing time
TpR = Proxy reply processing time
Tsr = Server request processing time
As noted in [8], store and forward proxies can have a negative effect on
accounting reliability. By sending a reply to the NAS without receiving
one from the accounting server, store and forward proxies fool the NAS
into thinking that the accounting request had been accepted by the
accounting server when this is not the case. As a result, the NAS can
delete the accounting packet from non-volatile storage before it has
been accepted by the accounting server. The leaves the proxy responsible
for delivering accounting packets. If the proxy involves moving parts
(e.g. a disk drive) while the NAS does not, overall system reliability
can be reduced. As a result, store and forward proxies SHOULD NOT be
used.
7.4. Transport layer proxies
With a transport layer proxy, the proxy may acts as an intermediary,
forwarding transport ACKs between the NAS and the Home Server. This type
of proxy effectively splices together the NAS-proxy and proxy-AAA server
connections into a single conection that behaves as though it operated
end-to-end. As a result, transport proxies will exhibit end-to-end self-
clocking. However, since these proxies need to operate at the transport
layer, they cannot be implemented purely as applications and examples of
AAA transport proxies are rare.
With a transport proxy, the sequence of events is as follows:
Time NAS Proxy Home Server
------ --- ----- -----------
0 Request
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------->
OTTnp + Tpr Request
------->
OTTnp + OTTph + Reply/ACK
Tpr + Tsr <-------
OTTnp + OTTph +
Tpr + Tsr + Reply/ACK
OTThp + TpR <-------
OTTnp + OTTph +
OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TdA ------->
OTTnp + OTTph +
OTThp + OTTpn +
Tpr + Tsr + Delayed ACK
TpR + TpD ------->
Key
---
OTT = One-way Trip Time
OTTnp = One-way trip time (NAS to Proxy)
OTTpn = One-way trip time (Proxy to NAS)
OTTph = One-way trip time (Proxy to Home server)
OTThp = One-way trip time (Home Server to Proxy)
TdA = Delayed ACK timer
Tpr = Proxy request processing time
TpR = Proxy reply processing time
Tsr = Server request processing time
TpD = Proxy delayed ack processing time
8. Security Considerations
General security considerations concerning TCP congestion control are
discussed in RFC 2581 [14].
9. IANA Considerations
This draft does not create any new number spaces for IANA
administration.
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10. Acknowledgments
Thanks to Allison Mankin of ISI, Barney Wolff of Databus, and Pat
Calhoun of Sun Microsystems for fruitful discussions relating to AAA
transport.
11. Authors' Addresses
Bernard Aboba
Microsoft Corporation
One Microsoft Way
Redmond, WA 98052
Phone: +1 (425) 936-6605
Fax: +1 (425) 936-7329
Email: bernarda@microsoft.com
12. Intellectual Property Statement
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13. Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
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on all such copies and derivative works. However, this document itself
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may not be modified in any way, such as by removing the copyright notice
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14. Expiration Date
This memo is filed as <draft-ietf-aaa-transport-01.txt>, and expires
October 1, 2001.
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